The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:    3GPP third generation partnership project    BLER block error rate    CDM code division multiplex(ed/ing)    CoMP coordinated multi-point    CQI channel quality indicator    CRS common (cell-specific) reference signal    CSI channel state information    CSI-RS channel state information reference signal    DL downlink    DM RS demodulation reference signal (user specific)    EPDCCH enhanced Physical Downlink Control Channel    E-UTRA evolved universal terrestrial radio access    eNB, eNodeB evolved node B/base station in an E-UTRAN system    E-UTRAN Evolved UTRAN (LTE)    HARQ hybrid automatic repeat request    LTE long term evolution    LTE-A long term evolution advanced    MCS modulation and coding scheme    MIMO multiple input multiple output    MU multi user    PRB physical resource block    PDCCH physical downlink control channel    PDSCH physical downlink shared channel    PUCCH physical uplink control channel    PUSCH physical uplink shared channel    PMI precoding matrix index    PMI-RS precoding matrix indicator reference signal (precoded RS for PMI selection)    RAN radio access network    RI rank index    RRC radio resource control    SIB system information block    SNR signal-to-noise ratio    SINR signal to interference plus noise ratio    TB transport block    TBS transport block size    TDD time division duplexing    TTI transmission time interval    UE user equipment    UL uplink    UTRAN universal terrestrial radio access network    WG working group
Multi-antenna MIMO techniques such as closed loop precoding and CoMP have received a lot of attention in the 3GPP for many years. A key design aspect in all closed loop MIMO related features is the channel state information (CSI) feedback consisting of one or more of CQI, PMI and RI and provided by the UE that helps the eNodeB in selecting the transmission parameters so that the data throughput is maximized.
The core part of any DL MIMO enhancements has been the precoding codebook design and the corresponding feedback signaling provided to the eNodeB by the UE. A codebook contains a set of precoding matrices that define the eNodeB antenna coefficients weights. Based on the DL reference signals such as CRS or CSI-RS the UE can identify the precoder from a given codebook (i.e., set of eNodeB antenna weights) that would maximize the signal quality and consequently the data throughput. The index of the precoder matrix (PMI) is fed back from the UE to the eNB as a recommendation for the precoder.
Moreover, DL MU-MIMO receives a considerable interest and will also be at the core of UE feedback enhancements: one key issue currently under consideration is how to design efficient and unified UE feedback in support of both DL SU- and MU-MIMO which are likely to be intrinsically tied together within the same DL transmission mode. When aiming at improving the system performance in LTE 3GPP Releases, the accuracy and granularity of the codebook and its suitability to the scenario of interest tends to become key issues in the discussions.
The problem related to codebook standardization is that the optimal codebook depends on the scenario, i.e., eNodeB antenna configuration. The factors impacting the optimal codebook design and the applicability of the codebook to different scenarios include at least the following considerations:                a number of eNodeB transmit antennas (e.g. 2, 4, 8 . . . ),        spacing between antenna elements (closely spaced, correlated antennas vs., e.g., distributed antenna systems),        polarity of the antennas (linear or cross-polarization), and        layout of the antenna configurations (uniform linear array, circular array, etc.).        
Recently there has been increasing discussion on new scenarios where the existing codebooks appear to be rather suboptimal including:                distributed antenna systems, where the spacing between some of the antennas may be significant,        massive MIMO where the number of antennas could be 16 or more,        vertical beamforming, where the precoding targets in addition to horizontal domain are also the vertical components, etc.        
A variety of deployment scenarios makes codebook standardization very difficult, since as discussed herein, the optimal codebook for one scenario may turn out to be a sub-optimal one for another scenario. The experience from the 3GPP standardization is that codebook standardization is very time consuming exercise that tends to lead to compromises that are not fully optimized for any scenario.
Besides, codebook based feedback results in a fixed granularity of CSI knowledge at the eNodeB side, since the UE can only select the recommended precoder (PMI) from a given codebook. That again makes it difficult to fit the CSI feedback to different types of environments. For example, in a typical flat radio environment a rough granularity is sufficient, while in an environment with rich multipath, a high granularity is needed. With a fixed size, the codebook cannot really result in a variable granularity and fit to different scenarios.
On the other hand, the LTE MIMO operation has been lately developing in the direction where UE specific demodulation reference signals (DM RS) are heavily utilized. With the DM RS the precoding that the eNodeB utilizes is transparent to the UE. The precoder that the eNodeB can use does not need to be signaled to the UE, instead the eNodeB may choose the antenna weights whatever the way it likes. The codebook is just for the purpose of the CSI feedback as a UE recommendation to the eNodeB. However, in practice the eNodeB has usually no other information to base the precoder selection on and hence it needs to settle for selecting the precoder from a standardized codebook, which may well be suboptimal.
In LTE 3GPP Release 10 downloadable codebooks were proposed by some companies. The core idea is that the eNodeB could configure any codebook for the UE to generate PMI/RI/CQI (e.g., there could be several standardized codebooks and the eNodeB would signal an indication of which one the UE should assume). The intention is to optimize the MIMO performance by using different codebooks for different scenarios. However, such proposal was not accepted in the 3GPP for Release.10. The main drawbacks are as follows:                1. UE PMI selection implementation must be flexible enough to allow all kind of codebooks configured by the eNodeB, which requires additional complexity;        2. Codebook configuration error reduces the robustness of the scheme;        3. Multiple scenarios result in a large number of codebooks to design thus requiring significant standardization effort.        